Install the witherable library, expose it in the prelude, and update all
call sites that are broken by that change.
This is a really nice library, and basically the ideal abstraction layer
for what it does.
Change-Id: I640e099318c1ecce0ad483bc336c379698bdab88
Reviewed-on: https://cl.tvl.fyi/c/depot/+/725
Reviewed-by: glittershark <grfn@gws.fyi>